P1669 — ABS Unit Expected

Causes

• ABS control module failure
• Loss of CAN/communication between ABS module and PCM/BCM
• Blown fuse or failed relay supplying the ABS module
• Open, shorted, corroded or disconnected wiring/connectors to ABS module
• Weak or discharged battery or poor charging system voltage
• Incorrectly programmed or incompatible replacement ABS module

Symptoms

• ABS warning lamp ON (often with Traction/Stability lamp)
• No ABS or traction control operation; loss of ABS braking functions
• Inability to communicate with the ABS module using a scan tool
• Possible related drivability or warning lamp behavior if modules exchange data
• Vehicle may enter reduced-stability or limp behavior depending on system design

What to check

• Read and record all stored and pending codes from all modules (ABS, PCM/BCM) and capture freeze frame data
• Verify battery voltage and starter/charging system condition (12.4–14.6 V expected with engine running)
• Visually inspect ABS module connectors and wiring for corrosion, damage, or water intrusion
• Check ABS system main fuse(s) and any relays; verify power and ground at ABS module harness pins
• Use a scan tool to attempt communication with the ABS module; note if the module is present and what messages appear
• Scan for related U-codes (CAN bus communication errors) and any other module reporting faults

Signal parameters

• ABS module supply voltage: battery voltage at module pins (approx. 12 V with key on; 13.5–14.5 V with engine running)
• Module ground: near 0 V (low milliohms continuity to chassis ground)
• CAN bus idle voltages: roughly 2.5 V on CAN High and CAN Low individually; differential pulses present during communication
• Wheel speed sensor outputs (if active): AC or variable frequency/voltage that increases with wheel speed (verify expected frequency/voltage at each wheel)
• ABS module message rate: CAN messages from ABS should be present when key on/engine running (verify with scan tool or oscilloscope)

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